Nitride semiconductors are excellent candidates as useful materials for short-wavelength light emitting devices because of their wide band gap. Among these, extensive research has been conducted on gallium nitride based compound semiconductors (GaN, AlGaN, GaInN, AlGaInN, and like GaN based semiconductors), and blue light emitting diodes (LED), and green LEDs have already been put to practical use. Furthermore, in order to increase the storage capacity of an optical disc apparatus, a semiconductor laser with its oscillation wavelength in the 400-nm band is in strong demand. For this reason, semiconductor lasers using GaN based semiconductors have attracted widespread attention, and are now approaching a level of practical use.
A device disclosed in Japanese Unexamined Patent Publication No. 2001-168442 (specification of U.S. Pat. No. 6,479,325) is known as a heretofore used GaN based semiconductor laser device. As shown in FIG. 3, this semiconductor laser device is formed with a junction-down configuration in which a pn-junction that includes a light-emitting active layer of the chip 210 is mounted to a sub-mount 220 that is connected to a heat sink 230 having a high heat diffusing property.
The chip 210 comprises an n-type contact layer 212, an n-type cladding layer 213, an active layer 214, a p-type cladding layer 215, a p-type contact layer 216, and a p-type electrode 217 layered in this order on the surface of a sapphire substrate 211. On the surface of the n-type contact layer 212, which has been partially removed and exposed by etching, an n-type electrode 218 is formed. As is clear from FIG. 3, the heights of the p-type electrode 217 and the n-type electrode 218 from the surface of the substrate 211 differ from each other, i.e., the p-type electrode 217 is in the higher position than the n-type electrode 218, for example, by approximately 3.5 μm.
The sub-mount 220 is formed on the surface of a supporting plate 221 by depositing lead electrode layers 222a and 222b, and solder films 223a and 223b. By pressing the chip 210 and the sub-mount 220 together while the solder films 223a and 223b are melted by heat, the p-type electrode 217 and the n-type electrode 218 in the chip 210 are joined to the lead electrode layers 222a and 222b, respectively. The rear surface of the supporting plate 221 is connected to the heat sink 230 via the solder film 222c. 
The solder films 223a and 223b on the front surface of the sub-mount 220 have thicknesses corresponding to the projected heights of the p-type electrode 217 and the n-type electrode 218, respectively, in the chip 210. For example, by setting the thickness of the solder film 223a shown on the left side of FIG. 3 to be approximately 3.5 μm and the thickness of the solder film 223b shown on the right side to be approximately 7 μm, a difference in level of approximately 3.5 μm is formed between the solder films 223a and 223b. This difference in level absorbs the effect of the difference in the projection heights between the p-type electrode 217 and the n-type electrode 218 in the chip 210.
However, in a semiconductor laser device having such a structure, there is a difference in the levels of the solder films 223a and 223b. Therefore, incomplete adhesion tends to occur particularly between the p-type electrode 217 and the solder film 223a, with the result that not only the reliability of the device is decreased but also the heat generated in the chip 210 is not efficiently diffused via the solder films 223a and 223b. 
On the other hand, when the solder films 223a and 223b that correspond to the p-type electrode and the n-type electrode 218 are the same thickness, problems arise due to the difference in the projection height between the p-type electrode 217 and the n-type electrode 218, wherein, as shown in FIG. 4(a), the solder film 223a that corresponds to the p-type electrode 217 is thinly expanded and the interval between the lead electrode layers 222a and 222b becomes unduly short or, as shown in FIG. 4(b), the chip 210 is mounted on the sub-mount 220 with an incline relative thereto, resulting in defective conductivity.